The tightly confined modal fields of single- or few-moded waveguiding lasers, superfluorescent sources, and amplifiers lead to a very strong interaction between any waveguided light and the active medium in the waveguiding core. Therefore, a comparatively small amount of gain medium is sufficient for providing the gain in these devices. Specifically, the gain for a given stored energy, as well as for a given absorbed pump power, is high. This is often beneficial, since it means that the pump power requirements for a given desired laser output power or amplifier gain can be low.
However, for several devices, this efficient interaction between mode and gain medium can be detrimental. The following example refers to certain types of amplifiers and lasers, but of course the skilled man will realise that the same or similar problems can occur in, for example, superfluorescent sources.
In a laser or amplifier, the achievable single-pass gain is limited to, say, 50 dB. The reason is that at this gain, a significant fraction of the pump power is converted to amplified spontaneous emission (ASE). A 10 dB higher gain results in approximately 10 dB more ASE, so at these gains, the extra pump power required to increase the gain further will be prohibitively high. Since the ASE limits the gain of the device, it also limits the energy stored in the gain media. This in turn obviously limits the amount of energy that a pulse can extract from the device. Consequently, the pulse energy that can be obtained from waveguiding lasers and amplifiers is limited. Instead, bulk (i.e., not waveguiding) lasers and amplifiers for which the extractable energy for a given gain can be several orders of magnitude lower are often employed to provide much higher pulse energies. However, the robustness and stability of bulk lasers is often inferior to waveguiding ones.
Moreover, the gain limit can also be problematic for lasers and amplifiers irrespective of whether the stored energy is a major concern, if the high gain appears at another wavelength than the desired one. The reason is that ASE (or lasing) at the gain peak will suppress the gain achievable at the desired wavelength, possibly to a value below what is required for a good amplifier or laser. This applies to all types of amplifiers and lasers.
Furthermore, in optically pumped lasers and amplifiers, a suitable interaction between the gain medium and the amplified or generated signal beam is not enough; also the interaction between the pump beam and the gain medium must be appropriate. However, in some types of lasers and amplifiers (typically cladding-pumped ones), the interaction with the pump beam is significantly smaller than the interaction with the signal beam. Then, for a device that efficiently absorbs the pump, the interaction with the signal beam will be much stronger than what is required. Unfortunately, this excess interaction is often accompanied by excess losses for the signal beam, since:
1. The scattering loss of an active medium is normally higher than it can be for a passive medium. For instance, rare-earth-doped fibers have scattering losses of, e.g., several orders of magnitude higher than standard, passive, single-mode fibers. PA1 2. A fraction of the active medium often has inferior properties. For instance, in Er-doped fibers, pairs of Er.sup.3+ -ions can form. These result in an unbleachable loss. The strong interaction then leads to a high loss. PA1 1. Lasers (e.g., Q-switched and gain-switched ones) and amplifiers in which it is desirable to store large energies. In these devices (as well as for so-called energy-storage devices in general), the reduced interaction leads to a larger stored energy before practical upper limits on the gain is reached. PA1 2. Optical amplifiers (typically semiconductor ones) for which even the energy of a single signal bit can be comparable to the stored energy. In those, already the amplification of a bit extracts enough energy to reduce the gain. This leads to four-wave mixing, cross-talk, and inter-symbol interference. This can be reduced with the higher stored energy that, for a given gain, accompanies the reduced interaction. PA1 3. Amplifiers and lasers in which an efficient pump absorption necessitates large amounts of gain media, which in prior-art devices leads to excessive small-signal absorption, background absorption, or excited static absorption at the operating wavelength, or excessive gain at another wavelength. A reduced interaction then leads to reduced losses. Moreover, a reduced interaction can reduce the gain at the undesired wavelength relative to that at the desired one, and thereby the problems associated with a too high gain at the wrong wavelength. This applies to lasers in which there is a significant unpumped loss (typically, reabsorption loss or out-coupling loss). These points are especially relevant for cladding-pumped devices. For example, to ensure sufficient pump absorption, the fiber may need to be so long that one or both of those problems arise. PA1 4. Saturable absorbers, in which the saturation power is otherwise too small. PA1 1. To reduce the susceptibility to so-called quenching and background losses, in particular for cladding-pumped devices. PA1 2. To obtain efficient emission at wavelengths otherwise inaccessible for devices where there is a significant unpumped loss, in particular for cladding-pumped devices. PA1 3. To improve the energy storage capabilities, for energy-storage devices. PA1 4. To reduce signal cross-talk and inter-symbol interference for signal amplifiers. PA1 5. To allow for a larger, predetermined, saturation power. PA1 1. An amplifying optical fiber in which the active medium is placed partly or wholly outside the waveguiding core, e.g., in a ring around the core. The gain medium can also reside inside the core in regions where the normalized modal intensity of the signal beam is small. The fiber can be made of a glass, partly doped with Pr.sup.3+, Tm.sup.3+, Sm.sup.3+, Ho.sup.3+, Nd.sup.3+, Er.sup.3+, or Yb.sup.3+, or a combination thereof, and it can be cladding-pumped. PA1 2. A cladding-pumped amplifier or laser in which the difference between the overlaps of the pump and signal beams with gain medium is substantially reduced compared to prior-art designs. PA1 3. A ring-doped optical fiber for high-energy pulse amplification or generation or other energy storage applications. The fiber can for instance be made of a glass, partly doped with Pr.sup.3+, Tm.sup.3+, Sm.sup.3+, Nd.sup.3+, Nd.sup.3+, Er.sup.3+, or Yb.sup.3+, or a combination thereof, and cladding-pumped. Moreover, the device can incorporate a longitudinally distributed saturable absorber to suppress the build-up of ASE. In one embodiment, the gain medium is a Yb.sup.3+ -sensitized Er.sup.3+ -doped glass, and the saturable absorber is an Er.sup.3+ -doped glass, and they are located so that the signal intensity is higher in the saturable absorber than in the gain medium. PA1 4. A Q-switched or gain-switched fiber laser based on an amplifying fiber with a relatively higher saturation energy combined with a saturable absorber fiber having a relatively lower saturation energy. The difference in saturation energy stems, at least to a significant part, from differences in the geometry of the fibers. The active media in the different fibers can be the same or different, and can for instance be a glass doped with a rare earth, e.g., Pr.sup.3+, Tm.sup.3+, Sm.sup.3+, Ho.sup.3+, Nd.sup.3+, Er.sup.3+, or Yb.sup.3 +, or a combination thereof. PA1 5. A ring-doped, cladding-pumped ytterbium-doped fiber for amplification or generation of light in the range 950 nm to 1050 nm. PA1 6. A ring-doped, cladding-pumped neodymium-doped fiber for amplification or generation of light in the range 850 nm to 950 nm. PA1 7. A ring-doped, cladding-pumped erbium-doped fiber for amplification or generation of light in the range 1450 nm to 1600 nm. PA1 8. An amplifying planar waveguide structure in which the active medium is placed partly or wholly outside the waveguiding core, thus interacting with the signal beam only where the normalized intensity of the modal field is small. The waveguide can be cladding-pumped. Moreover, the design can be specifically adapted to correspond to any of the fiber devices listed above. PA1 9. A semiconductor amplifier for signal amplification, in which the gain region is placed partly or wholly outside the waveguiding core, thus interacting with the signal beams only where their normalized modal intensities are small. Thereby, the saturation energy of the device will be increased, which subsequently reduces the inter-symbol interference and inter-wavelength cross-talk. PA1 10. A waveguiding structure with a saturable absorption, in which the absorbing medium is placed partly or wholly outside the waveguiding core, thus interacting with the signal beam only where its normalized modal intensity is small. PA1 1. It has not been one of the specific devices considered here. PA1 2. It has not used a single-moded or few-moded waveguiding core. PA1 3. It has not been a device in which the energy extraction results in cross-talk or inter-symbol interference. PA1 4. The control of the emission wavelength that we propose has not been obtained. PA1 5. The device has not substantially reduced the effect of losses at the signal wavelength. PA1 6. It has not been a cladding-pumped device. PA1 7. It has not been a device for high-energy pulses. PA1 8. It has not been an optical fiber doped with erbium or another rare-earth for high-energy pulses. PA1 9. The output of the device could not be launched into a standard single-mode fiber through splicing or butt-coupling, nor has the device allowed for an easy launch of signal light. PA1 10. The output beam has not been tightly confined. PA1 11. It has been a device doped in regions of the core where the modal intensity is large. PA1 12. It has been a device doped in a large area around the core (e.g., homogeneously in the cladding), hence rendering it inefficient for cladding-pumping. PA1 13. It has not been a fiber structure, or at least not an all-fiber structure. PA1 14. It has not been a solid-state device. PA1 15. The interaction length has been limited to a few centimeters. PA1 16. It has not been a high-gain device. PA1 17. It has not been a device pumped by an optical beam guided along the amplifying medium. PA1 18. It has not been possible to manufacture the device with standard manufacturing techniques for rare-earth doped fibers like MCVD and solution doping. PA1 19. The purpose of the design has not been to obtain a smaller interaction between the gain medium and the signal light than would otherwise be possible, nor have any substantial benefits of a substantially smaller interaction been proposed, discussed, or demonstrated.
3. The active medium in its amplifying state can also absorb light (so-called excited-state absorption, ESA). Again, a stronger interaction leads to more power lost through ESA.
Moreover, a bleachable medium (e.g., an unpumped gain medium with a ground-state absorption) can be used as a saturable absorber. An efficient interaction leads to a low saturation power. A reduced interaction leads to a higher saturation power, which can be more suitable for some applications, especially if the interaction, and hence the saturation power, can be controlled.
Clearly, although often beneficial, the tight confinement of the guided light is a problem for some devices.